Patients born with a single ventricle birth defect undergo a series of surgical procedures (three open heart surgeries) to directly connect the superior and inferior vena cava to the pulmonary arteries (Fontan), forming an approximately ‘plus’ shaped junction. In the plus shaped junction, the upper and lower vertical limbs are the superior and inferior vena cava and the horizontal limbs are the pulmonary arteries. In this regard, in a Fontan circulation, there is no right ventricle to power the pulmonary circulation. Both systemic and pulmonary circulations are powered by the single functional ventricle and, as such, these patients generally have a high risk of morbidity and mortality due to several factors including pulmonary hypertension, inability of the native single ventricle to support systemic and pulmonary circulations, and the need for a three-stage highly-invasive open heart surgical procedure.
Despite advances, palliative repair of functional single ventricle remains an enigmatic challenge. Late Fontan failure and attrition is becoming an increasingly problematic issue for which there is no primary therapy. Fontan failure is typically not the same as ventricular failure with systolic dysfunction. A majority (73%) of Fontan patients have normal systolic function and diastolic dysfunction. Indeed, the clinical manifestations of Fontan failure may be more a representation of decompensated systemic sequelae of Fontan physiology rather than that of primary ventricular failure.
Currently, the therapeutic options for failing Fontan patients are limited to medical therapy, surgical optimization, and mechanical circulatory support therapy. Medical therapy with diuretics, inotropes, and ACE-inhibitors provide some benefit, but only represent secondary therapy and do not fully restore long-term functional status. Surgical approaches to passively optimize total cavopulmonary connection (TCPC) to reduce power losses by 2-3 mmHg have been proposed. TCPC optimization may have tangible benefits but surgical modifications of the TCPC have yet to be applied clinically on a significant scale. Transplantation is a late surgical option, which has issues and concerns of its own, and does not currently represent an ideal long-term option. Patients with failing Fontan circulation have been implanted with a ventricular assist device (VAD) as a bridge to transplant. VADs unload the native ventricle, diminishing ventricular volume and external work, and augmenting the myocardial supply-demand ratio. While there have been reports of successful bridge to transplantation using VAD support in failing Fontan patients for brief periods of time, the results reported for post-cardiotomy bridge to transplantation have been poor. Additionally, pediatric bridge to transplant support of failing Fontan patients with a VAD is usually not successful.
A concerted effort is currently underway to develop cavopulmonary support devices (CSD) to power the Fontan circulation by delivering a modest pressure boost (approximately 5 mmHg) at the level of the total cavopulmonary connection (TCPC). In a univentricular Fontan circulation with preserved systolic function, CSD support will simultaneously decrease systemic venous pressure and increase ventricular preload. It would restore physiologic status to one more closely resembling more stable 2-ventricle physiology, in essence enabling clinical management of the patient as a “biventricular Fontan.” Previous CSD designs include two catheter based microaxial pumps in the vena cava(e), or a single percutaneous pump that can augment Fontan flow in all 4 axes of the TCPC without risk of venous pathway obstruction. The catheter-based designs, however, while minimally invasive, can only be used for short-term support (2-4 weeks) due to the risk of septicemia. Further, percutaneous access would restrict the mobility of the patients implanted with these devices. Right ventricular assist devices have been implanted for chronic support in failing Fontan patients but it requires take down of the TCPC and have limited long-term success. Further, all the CSD designs to date are blood contacting and have significant risk of thrombosis, requiring anticoagulation therapy.